Polyp (zoology)

[citation needed] The body of the polyp may be roughly compared in a structure to a sac, the wall of which is composed of two layers of cells.

[1] Polyps extend their tentacles, particularly at night, containing coiled stinging nettle-like cells, or nematocysts, which pierce, poison, and firmly hold living prey paralysing or killing them.

[2] Longitudinal muscular fibrils formed from the cells of the ectoderm allow tentacles to contract when conveying the food to the mouth.

[1] We can distinguish therefore in the body of a polyp the column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed the peristome, in the centre of which again is the mouth.

A polyp is an animal of very simple structure,[1] a living fossil that has not changed significantly for about half a billion years (per generally accepted dating of Cambrian sedimentary rock).

Anthozoan polyps, including the corals and sea anemones, are much more complex due to the development of a tubular stomodaeum leading inward from the mouth and a series of radial partitions called mesenteries.

[3] When eggs and sperm (gametes) are formed, they can produce zygotes derived from "selfing" (within the founding clone) or out-crossing, that then develop into swimming planula larvae.

[4] The name polyp was given by René Antoine Ferchault de Réaumur[6] to these organisms from their superficial resemblance to an octopus (French: poulpe, ultimately from Ancient Greek adverb πολύ (poly, "much") + noun πούς (pous, "foot")), with its circle of writhing arms round the mouth.

[1] 75% of the world's corals are threatened[7] due to overfishing, destructive fishing, coastal development, pollution, thermal stress, ocean acidification, crown-of-thorns starfish, and introduced invasive species.

Particularly, exposure to the insecticide profenofos and the fungicide MEMC have played a major part in polyp retraction and biomass decrease.

[10][11] There have been many experiments supporting the hypothesis that heat stress in Acropora tenuis juvenile polyps provokes an up-regulation of protein in the endoplasmic reticulum.